1. Field of the Invention
The invention relates to an electric high-voltage insulator made from plastic, comprising at least one glass fiber rod, at least one shield covering made from silicone rubber which surrounds the glass fibre rod and has concentric bulges arranged in the direction of the longitudinal axis of the insulator and bent in the shape of sheds in such a way that they form a convex top side and a concave or flat underside, as well as metal fittings at both insulator ends.
2. Description of Related Art
High-voltage insulators for overhead lines have been produced for a long time from ceramic, electrically insulating materials such as porcelain or glass. Alongside this, insulators containing a glass fiber core and a shield covering made from plastic in a composite design are gaining increasingly in importance, because they are distinguished by a series of advantages to which, in addition to a relatively low intrinsic weight, there also counts an improved mechanical resistance to projectiles from fire arms. The shield coverings of such composite insulators are in this case mostly constructed from cycloaliphatic epoxy resins, from polytetrafluoro-ethylene, from ethylene-propylene-diene rubbers or from silicone rubber.
By comparison with composite insulators made from other shield materials, and also by comparison with conventional insulators, composite insulators having a shield covering made from silicone rubber have the advantage that they have excellent insulating properties when used in areas having a highly polluted atmosphere. That is why silicone-rubber insulators are increasingly being used for the purpose of upgrading existing overhead lines having electric insulation problems, which result from atmospheric pollution, by exchanging the conventional insulators for composite insulators having a shield covering made from silicone rubber.
The tracking path required for operating the insulator can be obtained by the number and the diameter of the shields. In the case of very high atmospheric pollution in the area of use of the insulators, the tracking path of the insulators must be longer than in areas of use of low atmospheric pollution. In this case, physical limits, which are defined in the IEC Publication 815, exist for shed overhang and shield spacing. It is not possible for the purpose of obtaining a specific tracking path per insulator length to configure the screens with an arbitrarily large diameter, nor to arrange them arbitrarily close together. Natural limits are thus set here for flat shields.
It has therefore already been proposed to fit shields of plastic composite insulators on their underside with grooves for the purpose of lengthening the tracking path. Such insulators are presented, for example, in EP-A-0 223 777 or in DE-A 11 80 017. The insulators described there have not proved themselves in practice. Grooves on the shield undersides, such as are known from cap-and-pin insulators made from glass or porcelain, tend to fill up with dirt from the atmosphere. The self-cleaning properties of such insulators are poor, since the grooves cannot be washed out by the rain. High surface conductivities in fog are the consequence, with the result that such insulators made from conventional materials tend to electric flashovers, and such as are made from plastics are exposed to the risk of tracking or partial combustion. Consequently, because of the better self-cleaning power, use is made today of conventional and composite insulators having flat shields without grooves on the underside in areas of high atmospheric pollution. These insulators acquire their necessary tracking paths by large shield diameters and a correspondingly long insulator length which is, however, undesired.